Meeting this goal will demand reliable, current data and information about how much water is stored in large lakes, rivers and reservoirs around the world � which radar altimetry can provide.

In the past, hydrological information could often be difficult to obtain by ground-based gauge instruments due to the inaccessibility of the region, the sparse distribution of gauge stations or the slow dissemination of data due to national policy.

Radar altimetry can avoid these obstacles because it is located on satellites 800 to 1300 kilometres above the Earth and is able to measure large lakes' surface water height to two centimetres accuracy and rivers to ten centimetres by sending 1800 separate radar pulses over bodies of water per second and recording how long their echoes take to bounce back. In addition, these data are available in near-real time.

Today there are several teams in the world involved in radar altimetry over inland water, using satellite data from ESA, NASA and the French Space Agency (CNES). Hydrologists from each of these research teams met at the '15 Years of Progress in Radar Altimetry' symposium, organised by ESA and the French Space Agency (CNES) in Venice Lido, Italy, from 13-18 March 2006, to discuss the abilities of past and current altimeters for monitoring the Earth's changing inland water resources.

Professor Philippa A. Berry of the UK's Earth and Planetary Remote Sensing Laboratory (EAPRS) at De Montfort University said: "This field has evolved rapidly over the past two decades. Initial work over a handful of large targets has now expanded to the current capability to monitor thousands of river and lake heights worldwide."

Although designed for ocean studies, hydrologists undertook great efforts for radar altimetry to include precise river and lake level monitoring. Berry, for instance, led a River and Lake Level monitoring development project, under ESA contract, in an effort to track rivers and lakes to help manage water resources.

As a result, ESA launched a web-based demonstration in 2005 that allows for radar altimetry data of African rivers and lakes from its environmental satellite, Envisat, to be freely available worldwide in near-real time � within four days of measurements.

Following the 2002 World Summit, ESA partnered with UNESCO in starting up the TIGER initiative, which uses satellite data to manage water resources in Africa. ESA is currently presenting the TIGER initiative at the World Water Forum in Mexico City. The forum, being held from 16 to 22 March, has brought government, business, and non-government organisations together to discuss establishing a water-secure future and the UN goal of halving the number of people without drinking water by 2015.

The primary objective of TIGER is to help African countries overcome problems faced in the collection, analysis and dissemination of water related geo-information by exploiting the advantages of Earth Observation technology. More than 200 African water basin authorities, universities and other organisations have become involved in TIGER projects across the continent.

Although radar altimetry has been successful in measuring the height of rivers and lakes, scientists are looking for ways to improve the instrument in the future. More than 150 scientists from more than 20 countries have proposed an Earth Explorer hydrology mission called Water Elevation Recovery (WatER), which aims to determine how water storage varies in space and time.

According to Professor Doug Alsdorf of Ohio State University, conventional profiling altimetry � which uses a single antenna looking strait down at Nadir � limits the spatial and temporal sampling of water surface elevations, making it essential to obtain a two-dimensional sampling in order to capture the flow dynamics of highly complex waters.

To this end, WatER will use an interferometric altimeter, which will allow it to measure elevations over a 120 kilometre wide swath and therefore map the world's water bodies repeatedly in less than a week.

"WatER is designed to meet high priority targets for all nations and will provide essential data for the EU Water Framework Directive. WatER will meet the United Nations' call for a 'greater focus on water related issues,'" CNES'Dr. Nelly Mognard, lead scientist for the WatER proposal and part of Laboratoire d'Etudes en Geophysique et Oceanographie Spatiales, said at the symposium in Venice.

CryoSat-2, the next ESA radar altimetry mission, expected to launch in 2009, is designed to measure changing ice fields, but it will also contribute to monitoring water resources by acquiring samples of data from its new generation radar altimeter over inland water bodies upon request from scientists for experimental purposes.

A follow on to CryoSat, ESA Earth Explorer ice mission, which was lost at launch in 2005 due to an anomaly in the Russian launcher, CryoSat-2 will fly an enhanced radar altimeter instrument, called the Synthetic Aperture Interferometric Radar Altimeter (SIRAL) which will allow it to improve the resolution of the measurements by increasing the number of separate radar pulses it sends down to Earth every second from 1800 to up to 17 800. The experiment will demonstrate how to benefit from novel technologies to serve emerging science fields, such as hydrology, from space.

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